1. Field of the Invention
The invention generally relates to surface acoustic wave devices, and more particularly, to a surface acoustic wave device of a multimode type.
2. Description of the Related Art
Recently, filters formed by surface acoustic wave devices have been used in RF circuits of radio communications devices such as portable phones. The filters formed by the surface acoustic wave devices (SAW devices) may be used as a transmit filter, a reception filter and a duplexer in which the transmit filter and the receive filters are mounted in a single package. Various types of SAW filters are known, and are typically a ladder type SAW filter has SAW resonators arranged in a ladder formation, and a multimode type SAW filter. Generally, the ladder type SAW filter is used as a transmit filter, and the multimode type SAW filter is used as a receive filter.
As described in Japanese Patent Application Publication No. 2004-194269, the fundamental structure of the multimode type SAW filter has a pair of reflection electrodes formed on a piezoelectric substrate, and input and output interdigital transducers (IDTs) arranged between the reflection electrodes. When a drive voltage is applied across the input IDT, SAWs are propagated between the reflection electrodes, and multiple standing waves are produced between the reflection electrodes. A voltage based on the standing waves develops across the output IDT. The multimode type SAW filter thus formed functions as a bandpass filter.
FIG. 1A shows a unit of the multimode type SAW filter (fundamental structure), and FIG. 1B shows a cascade type in which two units are cascaded. A multimode type SAW filter 10 of the unit type includes an input IDT 12, output IDTs 14 and 16 arranged at opposing sides of the input IDT 12, and reflection electrodes 18 and 20 located further out than the output IDTs 14 and 16. Each of the input IDT 12, and the output IDTs 14 and 16 has a pair of comb-like electrodes in which electrode fingers are interleaved. The input IDT 12, the output IDTs 14 and 16, and the reflection electrodes 18 and 20 are formed by metal patterns formed on a piezoelectric substrate, which may be LN (lithium niobate) or LT (lithium tantalate). The impedance Fs of the unit type is designed so as to be equal to the characteristic impedance of a transmission line (for example, 50 Ω) connected to the SAW filter. The cascade type of multimode type SAW filter shown in FIG. 1B has two multimode type SAW filters that are cascaded. More specifically, two output IDTs of the multimode type SAW filter 10A are connected to two input IDTs of the multimode type SAW filter 10B through signal patterns 22 and 24, respectively. A comb-like electrode 12a of the input IDT of the multimode type SAW filter 10A and a comb-like electrode 12b of the output IDT of the multimode type SAW filter 10B are grounded. Since the comb-lie electrodes 12a and 12b are grounded, no signals are transmitted therebetween. The impedance Fi of the multimode type SAW filter 10A and the impedance Fo of the multimode type SAW filter 10B are both equal to the characteristic impedance of a transmission line connected to the SAW filter shown in FIG. 1B, and may be equal to, for example, 50 Ω (Fs=Fi=50 Ω).
FIG. 2 shows an arrangement in which three cascaded multimode type SAW filters, each shown in FIG. 1B, are connected in parallel.
The multimode type SAW filters shown in FIGS. 1A and 1B may be used as shown in FIG. 3A. A transmit filter Tx and a receive filter Rx are connected to an antenna Ant. The receive filter Rx is the multimode type SAW filter. A transmitted signal is output to the antenna Ant through the transmit filter Tx. A signal received via the antenna Ant is applied to a next-stage circuit through the receive filter Rx.
However, the inventors found out the following problem. If a Jammer (a frequency component in a desired receive frequency range) is received while the signal is being sent, a leakage component of the transmitted signal excites the multimode type SAW filter of the receive filter Rx together with the Jammer. This problem is shown in FIG. 3B, in which the horizontal axis (MHz) denotes the frequency and the vertical axis denotes the output power (dBm) of the receive filter Rx. As shown in FIG. 3B, only the Jammer appears at the output port of the receive filter Rx in the absence of the transmitted signal. In contrast, when a leakage component of the transmitted signal exists, power components appear at both sides of the peak of the Jammer resulting from the leakage component. The power components increase the intermodulation level and may degrade the single tone defense (STD), which is described in the standardized specification of portable phones. This problem occurs in not only an arrangement in which the transmit filter Tx and the receive filter Rx are formed by separate piezoelectric substrates but also an arrangement with a single piezoelectric substrate on which the transmit filter Tx and the receive filter Rx are formed.